Writing instrument



FIG. 1 is a side view of a writing instrument having a spiral shape (as a helical groove) in the back portion;

FIG. 2 is a front view of the writing instrument of FIG. 1;

FIG. 3 is an opposite side view;

FIG. 4 is a rear view; and,

FIG. 5 is a top view. 

The ornamental design for a writing instrument, as shown and described. 